1. Field of the Invention
This invention relates to nonaqueous acrylic resin dispersions and particularly to acrylic resin plastisols and organosols having low viscosity, low thixotropy and good filterability. This invention also relates to an improved process for preparing said dispersions and making shaped articles therefrom. Furthermore, this invention also relates to photosensitive acrylic resin dispersions containing photopolymerizable, ethylenically unsaturated compounds.
2. Description of the Prior Art
Certain polymeric resin dispersions referred to as plastisols are fluid mixtures, ranging in viscosity from pourable liquids to heavy pastes, obtained by dispersing polymeric resin particles in nonvolatile, nonaqueous liquid plasticizers, i.e., materials which are compatible with the polymer or resin and increase its workability and flexibility but are not solvents for the polymer or resin under ordinary conditions of storage. When the plastisol has been formed into a desired shaped, e.g., by molding or coating, it can be heated to coalesce the resin or polymer particles and the nonvolatile liquid constituent, thereby forming a homogeneous mass. It is known to add volatile diluents to plastisol dispersions to modify their viscosity and to achieve desirable handling characteristics in coating or other forming operations. When the dispersion contains no more than 10% volatile diluent, it is still regarded as a plastisol; when the volatile diluent content exceeds 10%, the dispersion is defined as an "organosol", H. A. Sarvetnick, "Plastisols and Organosols", Van Nostrand Reinhold Company, New York, New York (1972), page 201.
Although theoretically, by appropriate choice of ingredients, any polymeric resin can be made into a plastisol, in practice both the commercial use and the technical literature have focused almost exclusively on the use of polyvinyl chloride in forming plastisols, to the extent that "plastisol" is cross-referenced to "polyvinyl chloride" in Chemical Abstracts and in many textbooks plastisols are described as suspensions of polyvinyl chloride resin.
The pre-eminence of polyvinyl chloride in the practical and technical plastisol art results from the fact that many polymers with otherwise highly useful and desirable characteristics can be dispersed in suitable plasticizers only with great difficulty to give dispersions that have impractically high initial viscosities or very short storage life or both. Among these other desirable materials are various acrylic polymers and copolymers, which are attractive for a wide variety of uses because of their clarity, toughness, durability, availability, relatively low cost, ease of forming into desired final product by a number of methods, and the like. Despite their many desirable attributes, these polymers and copolymers have found no practical application in the plastisol and organosol arts because it has not been known heretofore how to prepare dispersions with suitably low and stable viscosities.
It has now been found that plastisols and organosols with very acceptable rheological properties can be prepared by dispersing a binder powder that comprises a major amount of an acrylic homopolymer or copolymer in a liquid medium that contains not only the usual nonvolatile plasticizer but also a volatile solvent for the resin, and thereafter removing the volatile solvent either before forming the dispersion into the desired shaped article or before the final coalescing step.
It is known from the prior art, concerned almost exclusively with polyvinyl chloride resins, to add organic diluents to plastisol and organosol dispersions, as in U. S. Pat. No. 2,379,236 and in the Sarvetnick textbook referred to above. However, in these prior art teachings, only the nonsolvent component of the diluent is said to be volatile, whereas any solvent component that may be present is not only nonvolatile or at least less volatile than the nonsolvent component, but is also taught to have negligible solvent activity on the resin at room temperature. Furthermore, these references teach that the incorporation of the solvent component operates to cause gelation and an irreversible increase in the viscosity of the dispersion. In contradiction of these prior art teachings, it has surprisingly been found, according to the present invention, that incorporation of a volatile solvent component in acrylic resin dispersions leads to a permanent reduction in viscosity and thixotropy and a permanent improvement in filterability, even when the volatile solvent is present in the dispersion for only a relatively short period of time and is then removed, e.g., by distillation or evacuation.
Several applications relating to plastisols and organosols have been filed in the United States in the name of Applicant on Mar. 22, 1977. One is Ser. No. 780,085, now abandoned which relates to a thermally coalescible acrylic plastisol or organosol dispersion having particles of a single phase, surfactant free, random acrylic polymer or copolymer, containing at least 80% by weight of acrylic units dispersed in a surfactant-free nonvolatile compatible liquid plasticizer. A nonvolatile photopolymerizable ethylenically unsaturated monomeric compound and photoinitiator can be present in the liquid phase of the dispersion. Another application is Ser. No. 780,086, now U.S. Pat. No. 4,176,028, which relates to a thermally coalescible nonaqueous dispersion of a random polymer or copolymer which comprises an organic polyelectrolyte that contains at least 1% by weight of an ionizable monomer or comonomer dispersed in a nonvolatile compatible liquid. A photopolymerizable monomer and photoinitiator can be present in the liquid phase. Still another application, Ser. No. 780,088, now U.S. Pat. No. 4,125,700, relates to a process for the preparation of plastisols and organosols which can have photopolymerizable monomers and photoinitiators present.